Ralf Busch

Viscosity of the supercooled liquid and relaxation at the glass transition of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy

Abstract The flow and relaxation of the Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass forming alloy was investigated in the supercooled liquid state and the glass transition region using parallel plate rheometry, three-point beam bending as well as differential scanning calorimetry. The results indicate that this bulk metallic glass former is a strong liquid that, kinetically, behaves in a similar way as a silicate melt. The relaxation of the viscosity and specific heat capacity was investigated in isothermal experiments and with constant heating rates. The studies reveal that the relaxation of the viscosity into its equilibrium state is directly related to the calorimetric glass transition. The observed calorimetric glass transition in this strong glass former is of a purely kinetic nature and a thermodynamically metastable amorphous state cannot be observed in this bulk metallic glass forming liquid on a laboratory time scale. The heating rate dependence of the calorimetric glass transition reflects the fragility of the liquid.

Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites

The Zr[sub 57]Nb[sub 5]Al[sub 10]Cu[sub 15.4]Ni[sub 12.6] bulk metallic glass forming liquid is reinforced with WC, SiC, W, or Ta particles. Structure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remains amorphous after adding up to 20 vol.% of particles. The reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy and electron microprobe. The mechanical properties of the composites are studied in compression and tension. The influence of the introduced particles on the thermal stability of the matrix as well as on the mechanical properties is discussed.

Thermodynamics and kinetics of the Mg65Cu25Y10 bulk metallic glass forming liquid

The thermodynamics and kinetics of the bulk metallic glass forming Mg65Cu25Y10 liquid were investigated using differential scanning calorimetry and three-point beam bending. The experiments lead to the determination of the thermodynamic functions as well as the viscosity of the supercooled liquid. The viscosity shows a temperature dependence, which is consistent with that of a strong glass similar to Zr–Ti–Cu–Ni–Be bulk metallic glasses or sodium silicate glasses. This contrasts with more fragile conventional metallic glass formers or pure metals. The relatively weak temperature dependence of the thermodynamic functions of the supercooled liquid is related to these sluggish kinetics in the supercooled liquid. Entropy, viscosity, and kinetic glass transition are compared in the frameworks of the fragility concept and the Adam–Gibbs theory. Strong liquid behavior retards the formation of crystals kinetically and thermodynamically.

Thermodynamics and kinetics of the undercooled liquid and the glass transition of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy

Differential scanning calorimetry (DSC) was used to determine the thermodynamic functions of the undercooled liquid and the amorphous phase with respect to the crystalline state of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5bulk metallic glass forming alloy. The specific heat capacities of this alloy in the undercooled liquid, the amorphous state and the crystal were determined. The differences in enthalpy, ∆H, entropy, ∆S, and Gibbs free energy, ∆G, between crystal and the undercooled liquid were calculated using the measured specific heat capacity data as well as the heat of fusion. The results indicate that the Gibbs free energy difference between metastable undercooled liquid and crystalline solid, ∆G, stays small compared to conventional metallic glass forming alloys even for large undercoolings. Furthermore, the Kauzmann temperature, TK, where the entropy of the undercooled liquid equals to that of the crystal, was determined to be 560 K. The Kauzmann temperature is compared with the experimentally observed rate-dependent glass transition temperature, Tg. Both onset and end temperatures of the glass transition depend linearly on the logarithm of the heating rate based on the DSC experiments. Those characteristic temperatures for the kinetically observed glass transition become equal close to the Kauzmann temperature in this alloy, which suggests an underlying thermodynamic glass transition as a lower bound for the kinetically observed freezing process.

Diffusion mechanisms in metallic supercooled liquids and glasses

The mechanisms of atomic transport in supercooled liquids and the nature of the glass transition are long-standing problems. Collective atomic motion is thought to play an important role in both phenomena. A metallic supercooled liquid represents an ideal system for studying intrinsic collective motions because of its structural similarity to the “dense random packing of spheres” model, which is conceptually simple. Unlike polymeric and network glasses, metallic supercooled liquids have only recently become experimentally accessible, following the discovery of bulk metallic glasses. Here we report a 9Be nuclear magnetic resonance study of Zr-based bulk metallic glasses in which we investigate microscopic transport in supercooled liquids around the glass transition regime. Combining our results with diffusion measurements, we demonstrate that two distinct processes contribute to long-range transport in the supercooled liquid state: single-atom hopping and collective motion, the latter being the dominant process. The effect of the glass transition is clearly visible in the observed diffusion behaviour of the Be atoms.

Decomposition and primary crystallization in undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 melts

Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glasses were prepared by cooling the melt with a rate of about 10 K/s and investigated with respect to their chemical and structural homogeneity by atom probe field ion microscopy and transmission electron microscopy. The measurements on these slowly cooled samples reveal that the alloy exhibits phase separation in the undercooled liquid state. Significant composition fluctuations are found in the Be and Zr concentration but not in the Ti, Cu, and Ni concentration. The decomposed microstructure is compared with the microstructure obtained upon primary crystallization, suggesting that the nucleation during primary crystallization of this bulk glass former is triggered by the preceding diffusion controlled decomposition in the undercooled liquid state.

Equilibrium viscosity of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass-forming liquid and viscous flow during relaxation, phase separation, and primary crystallization

Abstract The flow behavior of the supercooled Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 bulk metallic glass-forming liquid is studied in isothermal three-point beam-bending experiments. The experiments lead to the determination of the equilibrium viscosity as a function of temperature. Comparison with other glass-forming liquids shows that the Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy is a strong liquid, similar to sodium silicate liquids. Flow measurements during phase separation and subsequent formation of crystals embedded in a non-crystalline matrix reveal a dramatic slowdown of the kinetics of the matrix that is expressed in an increase of the viscosity by several orders of magnitude.

The effect of silicon on the glass forming ability of the Cu47Ti34Zr11Ni8 bulk metallic glass forming alloy during processing of composites

Composites of the Cu47Ti34Zr11Ni8 bulk metallic glass, reinforced with up to 30 vol % SiC particles are synthesized and characterized. Results based on x-ray diffraction, optical microscopy, scanning Auger microscopy, and differential scanning calorimetry (DSC) are presented. During processing of the composites, a TiC layer forms around the SiC particles and Si diffuses into the Cu47Ti34Zr11Ni8 matrix stabilizing the supercooled liquid against crystallization. The small Si addition between 0.5 and 1 at. % increases the attainable maximum thickness of glassy ingots from 4 mm for Cu–Ti–Zr–Ni alloys to 7 mm for Cu–Ti–Zr–Ni–Si alloys. DSC analyses show that neither the thermodynamics nor the kinetics of the alloy are affected significantly by the Si addition. This suggests that Si enhances the glass forming ability by chemically passivating impurities such as oxygen and carbon that cause heterogeneous nucleation in the melt.

Metallic glass formation in highly undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 during containerless electrostatic levitation processing

Various sample sizes of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 with masses up to 80 mg were undercooled below Tg (the glass transition temperature) while electrostatically levitated. The final solidification product of the sample was determined by x-ray diffraction to have an amorphous phase. Differential scanning calorimetry was used to confirm the absence of crystallinity in the processes sample. The amorphous phase could be formed only after heating the samples above the melting temperature for extended periods of time in order to break down and dissolve oxides or other contaminants which would otherwise initiate heterogeneous nucleation of crystals. Noncontact pyrometry was used to monitor the sample temperature throughout processing. The critical cooling rate required to avoid crystallization during solidification of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy fell between 0.9 and 1.2 K/s.

Thermodynamics of Cu47Ti34Zr11Ni8,Zr52.5Cu17.9Ni14.6Al10Ti5 and Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloys

The differences in the thermodynamic functions between the liquid and the crystalline states of three bulk metallic glass forming alloys, Cu47Ti34Zr11Ni8, Zr52.5Cu17.9Ni14.6Al10Ti5, and Zr57Cu15.4Ni12.6Al10Nb5, were calculated. The heat capacity was measured in the crystalline solid, the amorphous solid, the supercooled liquid, and the equilibrium liquid. Using these heat capacity data and the heats of fusion of the alloys, the differences in the thermodynamic functions between the liquid and the crystalline states were determined. The Gibbs free energy difference between the liquid and the crystalline states gives a qualitative measure of the glass forming ability of these alloys. Using the derived entropy difference, the Kauzmann temperatures for these alloys were determined.